Self-coplanarity bumping shape for flip chip

ABSTRACT

A stud bump structure for electrical interconnection between a pair of members includes a base portion, and a stem portion. The base portion is affixed to a pad or trace in one of the pair of members to be interconnected (such as an integrated circuit chip), and the stem end is configured to contact a metal pad on the other member (such as a printed circuit board) to complete the interconnect. According to the invention, the stem end is truncated to form a transverse plane, and the stem is more compliant than the base. Also, a method for forming a stud bump on a contact surface, includes forming a bump base portion on the surface, drawing out a generally conical tail from a top of the base, and truncating the tail to form a stem portion having a planar transverse top surface and having a length from the top of the base portion to the top surface. In some embodiments the tail portion, at least, of the stud bump is formed using a wire bonding tool. Also, a method for forming an interconnect between a first member and a second member of an electronic package includes providing one of the members with the stud bumps of the invention and then bringing the corresponding bumps and pads together in a bonding process, the compliance of the stems portions of the bumps accommodating the variance from coplanarity of the pad surfaces.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from Provisional Application No.60/272,240, filed Feb. 27, 2001.

BACKGROUND

[0002] This invention relates to forming electrical interconnectionbetween an integrated circuit chip and a substrate. In particular, theinvention relates to interconnecting an integrated circuit chip with thesubstrate in a flip chip semiconductor device package.

[0003] Interconnection between an integrated circuit chip and asubstrate such as a printed circuit substrate is commonly formed duringassembly of the device package. In a commonly used interconnectionmethod, gold bumps are mounted on the integrated circuit chip in anarrangement corresponding to the arrangement of metal contact pads onthe substrate. During package assembly the chip and the substrate areapposed with the corresponding bumps and pads aligned; then the chip andsubstrate are brought together under conditions (temperature, pressure,untrasonic vibration) that promote bonding of the bumps onto the metalpads.

[0004] The surfaces of the integrated circuit chip and the substrate arenot uniformly flat. The substrate, which may be formed of an organicpolymer, may have a particularly nonflat surface, particularly where thesubstrate is fabricated using a lower cost process. The substratesurface may be warped or may otherwise have irregularities. Accordingly,the surfaces of the metal contact pads on the substrate may benoncoplanar. As a result, when the surfaces of the chip and thesubstrate are apposed, the various metal pad surfaces are at differentdistances from the corresponding contact surfaces of the chip. Becauseof this irregularity, when the chip and the substrate are broughttogether, some of the bumps may fail to make good contact with theircorresponding pads, resulting in a nonfunctioning package.

[0005] There is a continual demand in industry for reduced size insemiconductor packages. As packages are made smaller, the interconnectstructures are also made smaller and the clearance between the chip andthe substrate becomes narrower. Where the clearance between the chip andthe substrate is very narrow even comparatively slight noncoplanaritiesof the substrate surface become significant, and can result in anunacceptable rate of interconnect failure during package construction.

[0006] There is a need for improved reliability in construction ofrobust interconnects in electronic chip package assembly.

SUMMARY

[0007] According to the invention, a stud bump structure includes a baseportion, and a stem portion. The base portion is affixed to a pad ortrace in one of a pair of members to be interconnected (such as anintegrated circuit chip), and the stem end is configured to contact ametal pad on the second member (such as a printed circuit board) tocomplete the interconnect. According to the invention, the stem end istruncated to form a transverse plane, and the stem is more compliantthan the base. The difference in compliance may be provided by formingthe stem with a smaller diameter than that of the base. Or, thedifference in compliance may be provided by forming the stem of a morecompliant material than that of the base. The first and second membersare apposed, with the stud bumps on the first member in alignment withthe corresponding metal pads on the second member. The members are movedtoward one another under conditions that favor bonding of the stud bumpstems onto the metal pads. Because the surfaces of the members are notuniformly flat, and the surfaces are moved toward one another certain ofthe stems contact their corresponding pads before others do; these stemsare deformed as the surfaces are moved closer together and still more ofthe stems make contact with their corresponding pads; eventually all ofthe stems have contacted and bonded with their corresponding pads. Thestud bump bases, being less compliant than the stems, substantiallymaintain their shape during stem deformation, and the bases can providea stop, preventing further movement together of the first and secondmembers and ensuring that a minimum distance between the surfaces (apartfrom the bumps and pads) is maintained. The extent to which the stemportion of the stud bump may be shortened in the deformation processprovides a practical tolerance for noncoplanarity of the metal padsurfaces.

[0008] The stud bump structure according to the invention isparticularly useful in interconnects for very thin flip chip packages,in which the first member is an integrated circuit chip, and secondmember is a substrate such as a printed circuit substrate.

[0009] In one general aspect the invention features a stud bumpstructure comprising a base portion, affixed to one member of aninterconnect pair (such as a integrated circuit chip), and a stemportion, in which the stem portion has a transverse planar top surfaceconfigured to contact a metal pad on a second member of an interconnectpair (such as a printed circuit board substrate). The stem portion ismore compliant than the base portion, so that under pressure appliedagainst the top surface toward the base, under conditions that promotebonding of the stem with the pad, the stem portion deforms and becomesshorter, while the base portion remains substantially undeformed.

[0010] In some embodiments the bump base portion comprises a firstmaterial and the stem portion comprises a second material, and thesecond material is more compliant than the first material. Because thesecond material is more compliant than the first, pressure appliedagainst a top surface of the stem portion, toward the base, can (underconditions that promote bonding of the stem with the pad) result indeformation of the stem portion, so that it becomes progressivelyshorter, while the base portion remains substantially undeformed. Insome embodiments the first material comprises nickel or a nickel alloyor copper or a copper alloy; and the second material comprises a solderor gold or a gold alloy. In embodiments in which to second materialcomprises a solder, the conditions that promote bonding of the stem withthe pad include applying sufficient heat to fuse the solder with thepad.

[0011] In some embodiments the bump base portion comprises a materialhaving a compliance characteristic either the same as or similar to ordifferent from that of the material of the stem, and may for examplecomprise substantially the same material as the stem portion. In suchembodiments, at least the top surface of the stem has a smaller diameterthan the base, so that although there is no difference in the complianceof the material, the stem is more compliant than the base because of itssmaller diameter. A preferred material is gold, or a gold alloy.

[0012] The extent to which the stem may shorten as it is deformed duringthe bonding process establishes a limit on the range of noncoplanaritythat may be tolerated for the given members that are beinginterconnected. Accordingly—assuming that the stem top surfaces of allthe bumps on the first member are substantially planar—this shorteningcapacity (which may be referred to as the “Z-level adjustment”) shouldbe selected to be greater than the maximum range of noncoplanarity forthe respective pads on the second member. This ensures in bonding of anygiven pair that shortening of the stem portion of the bump that contactsthe pad that is nearest the first member surface will allow for goodcontact of a bump with the pad that is farthest from the first membersurface. In other words, the Z-level adjustment provides foraccommodation of the maximum degree of noncoplanarity of the pads, orbetween the first and second members.

[0013] In another general aspect the invention features a method forforming a stud bump on a contact surface, by forming a bump base portionon the surface, drawing out a generally conical tail from a top of thebase, and truncating the tail to form a stem portion having a planartransverse top surface and having a length from the top of the baseportion to the top surface. In some embodiments the tail portion, atleast, of the stud bump is formed using a wire bonding tool, and theconical tail is dimensioned so that after truncating, the resulting topsurface of the stem portion has a diameter at least about the same as aspecified wire diameter and no greater than the diameter of the base,more usually at least about 2 times a specified wire diameter and nogreater than the diameter of the base. The planar transverse stem topsurface is an important feature according to the invention, as itimproves the contact of the planar top surface with the metal pad duringthe bonding process, and can more reliably result in a more robustinterconnect. In some embodiments the truncating includes chemicalmechanical polishing.

[0014] During the bonding process, the base portion of the stud bump maybe displaced or may expand to some extent. Although any such deformationis insubstantial in relation to the deformation of the stem portion ofthe stud bump, contact of the stud bump base with the surroundingpassivation material on the first member might cause some disruption ofthe passivation material, or some degradation of the reliability of thepassivation material, owing to pressure or thermal stress at the pointof contact. In order to mitigate such effects, in some embodiments thebase portion of the stud bump has a diameter no greater than 0.85 timesthe diameter of the opening in the passivation to the bond pad on thefirst member.

[0015] In another general aspect the invention features a method forforming an interconnect between a first member and a second member of anelectronic package, by: providing a plurality of stud bumps on contactsites at specified locations on the first member, the stud bumps eachhaving a base portion and a stem portion that is more compliant than thebase portion, the stem portion having a transverse planar top surface;providing a second member having a plurality of bonding pads atspecified locations on a surface of the second member, the respectivespecified locations on the first and second members corresponding;apposing the first and second members such that the corresponding bumpsand pads are aligned; and bringing the first and second members togetherunder conditions that promote bonding of the pumps onto the respectivemetal pads.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a diagrammatic sketch in a sectional view showing aconventional stud bump interconnect structure.

[0017]FIG. 2 is a diagrammatic sketch in a sectional view of a flip chippackage employing stud bumps as in FIG. 1, showing interconnect failuresowing to variation in coplanarity between the die and the substrate.

[0018]FIG. 3 is a diagrammatic sketch in a sectional view showing anembodiment of a stud bump interconnect structure according to theinvention.

[0019]FIG. 4 is a diagrammatic sketch in a sectional view of a flip chippackage employing stud bumps according to the invention, showingimproved interconnection between the die and the substrate.

DETAILED DESCRIPTION

[0020] The invention will now be described in further detail byreference to the drawings, which illustrate alternative embodiments ofthe invention. The drawings are diagrammatic, showing features of theinvention and their relation to other features and structures, and arenot made to scale. Particularly, certain thickness dimensions areexaggerated in the FIGs. for purposes of illustration. For improvedclarity of presentation, in the FIGs. illustrating embodiments of theinvention, elements corresponding to elements shown in other drawingsare not all particularly renumbered, although they are all readilyidentifiable in all the FIGs.

[0021] Turning now to FIG. 1, there is shown diagrammatically in asectional view generally at 10 a conventional stud bump interconnectstructure affixed to a contact surface at a selected contact site on anintegrated circuit chip. Only a portion of the integrated circuit chipis shown, particularly, a contact site 12 which may for example be aconductive line such as an aluminum trace, and a passivation layer 14having an opening 13 exposing a surface 11 of the conductive line. Thestud bump 16 is formed in electrically conductive contact with theconductive line. The stud bump has a flattened spheroidal shape, as aslightly flattened sphere or a filled torus, and may as a manufacturingartifact have a vestigial slightly raised portion 18 that is a remnantof the process of forming the bump. The stud bump has an overall height15 above the conductive line surface 11 that includes the height 19 ofthe slightly raised upper portion 18 and the height 17 of the flattenedspheroidal bump 16 itself. The height 17 of the bump 16 differs fordifferent package configurations, and particularly it is less forpackages having a narrower clearance between the chip and the substrateto which the chip is to be interconnected. Where a vestigial slightlyraised portion 18 is present on the bump, its height 19 is typicallyless than about one quarter of the base height; for example, a typicalconventional bump having a height 17 about 25 microns will have noraised portion 18 or, if a raised portion is present it typically has aheight 19 less than about 6 microns.

[0022] Conventionally, a stud bump 16 is formed by an adaptation of awire bonding process using a wire bonding tool. Particularly, a wirebonding tool configured for forming a gold or gold alloy wire bondhaving a specified wire diameter is employed to form a roughly spherical(globular) wire end, which is contacted with the surface of theconductive line under conditions of force and temperature that promotebonding of the globular wire end onto the conductive line surface, andresulting in some degree of flattening of the globular wire end.Thereafter, the wire bonding tool is pulled away, and the wire portionis trimmed off close to the globular bump 16, leading the residualportion 18.

[0023]FIG. 2 is a diagrammatic sketch in a sectional view showinggenerally at 20 a result of a failed attempt to form an adequateinterconnection between an integrated circuit chip, having conventionalbumps as in FIG. 1, with a substrate having a corresponding arrangementof metal pads on a nonflat surface. In FIG. 2, an integrated chipindicated at 22 is provided with a number of conventional bumps, forexample 21 and 23, affixed to contact sites, not shown in FIG. 2, in theintegrated chip surface; and a substrate 24 is provided on a surface 26with a number of metal pads, for example 25 and 27. The substratesurface 27 is, as may be expected as a result of ordinary fabricationprocesses, not flat; and, accordingly, the upper surfaces 28 of themetal pads, for example 25, 27, are noncoplanar. Particularly, when thesurfaces of the integrated circuit chip and the substrate are apposed,certain of the pads (for example pad 25) are, because of thenoncoplanarity, nearer the apposed integrated circuit chip surface thanothers (for example pad 27). As a result, when the integrated chip 22and the substrate 24 are moved toward one another during theinterconnect bonding stage of the package assembly process, contact ofthese more proximate pads (e.g., 27) with their respective bumps (e.g.,23) restricts further movement of the integrated chip 22 and thesubstrate 24 toward one another, and interferes with effective contactof the more distant pads (e.g., 25) with their respective bumps (e.g.,21), preventing the formation of robust interconnect at these points.

[0024] Improved interconnect formation on substrates having noncoplanarpad surfaces is provided according to the invention by employing studbumps each having a base portion and a stem portion, in which the baseportion is affixed to a conductive pad or trace on the integratedcircuit chip and the stem portion is configured to contact a conductivepad on the substrate, and in which the stem portion is more compliantthan the base. An embodiment of a stud bump according to the inventionis shown by way of example in FIG. 3. The stud bump according to theinvention includes base portion 36 and stem portion 38. Base portion 36is affixed to a surface 11 on a conductive line 12 exposed within anopening 13 in a passivation material 14. Stem portion 38 is shapedgenerally as a truncated cone having a transverse planar top surface 39.

[0025] A stud bump according to the invention can conveniently be formedby an adaptation of a wire bonding process using a wire bonding tool.Particularly, a wire bonding tool configured for forming a gold or goldalloy wire bond having a specified wire diameter is employed to form aroughly spherical (globular) wire end, which is contacted with thesurface of the conductive line under conditions of force and temperaturethat promote bonding of the globular wire end onto the conductive linesurface, and resulting in some degree of flattening of the globular wireend. This somewhat flattened globular wire end constitutes the baseportion 36 of the bump. Thereafter, the wire bonding tool is pulled awayat a specified rate to form a roughly conical tail, generally asdescribed for example in U.S. Pat. No. 5,874,780, herein incorporated byreference. Then the tail is trimmed, and the transverse planar topsurface 39 is formed, for example by chemical mechainical polishing,resulting in the truncated roughly conical stem portion 38.

[0026] The stem portion 38 of the stud bump according to the inventionis more compliant than the base portion 36.

[0027] The difference in compliance may result from a difference inmaterial selected for forming the stem portion and the base portion.That is, the stem portion may be made of a material than a softer thanthat of the base portion under the conditions of interconnect bondformation. The base portion can be formed of nickel, or copper, or anickel or copper alloy, for example; and the stem portion can be formedof a solder, or of gold or a gold alloy. Other combinations of materialsmay be used, selected for their having relatively different compliancesunder interconnect bond formation conditions. Where a solder is used asthe stem portion, for example, the interconnect bond formationconditions will include heat treatment sufficient to melt and fuse thesolder onto the pad, but not so high as to permit substantialdeformation of the base portion. And where gold is used as the stemportion, for example, the interconnect bond formation conditions willinclude application of force sufficient to compress and shorten the stemportion, but not so great as to deform the base portions.

[0028] The difference in compliance may additionally or alternativelyresult from a difference in dimensions of the base and stem portions, asshown by way of example in FIG. 3. Particularly, for example, thediameter 33 of the surface 39 of the stem portion is significantly ifthe less than the diameter 31 of the base, as is the widest diameter ofthe stem portion, where it joins the base. A compressive force imposedagainst the top surface 39 of the stem portion and in the direction ofthe stem portion axis can, up to a limit of force, deform and shortenthe stem without significantly deforming the base.

[0029] Because the base portions are substantially not compressible in adirection normal to the surfaces of the chip and of the substrate, theheight 32 of the base portion establishes a practical lower limit on theclearance between the chip and the substrate surfaces. Similarly,although the stem portion is not infinitely compressible in a directionnormal to the surfaces of the chip and of the substrate, the height 34of the stem portion establishes a practical upper limit on the extent ofpad surface noncoplanarity that can be tolerated.

[0030]FIG. 4 diagrammatically illustrates generally at 40 a successfulrobust interconnect of an integrated chip with a nonflat substrateemploying stud bumps according to the invention. Here integrated chip 22is provided with a number of stud bumps according to the invention, eachhaving a base portion 36 and the stem portion 38, affixed to contactsites, not shown in FIG. 4, in the integrated chip surface. A substrate24 is provided on a surface 26 with a number of metal pads, for example25 and 27. The substrate surface 27 is, as may be expected as a resultof ordinary fabrication processes, not flat; and, accordingly the uppersurfaces 28 of the metal pads, for example 25, 27, are noncoplanar. Themaximum degree of noncoplanarity is shown has the dimension 29 in FIG.4; that is the difference between the heights, in relation to the chipsurface, of the pad surfaces 28 of the nearest pad 27 and of thefarthest pad 25. As FIG. 4 shows, the stem portions 38 of the stud bumpsaccording to the invention are made with sufficient height 34 and aremade sufficiently more compliant than the base portions 36, that thefarthest pad 26 has formed a robust bond with the top surface of thecorresponding stud bump stem, while the nearest pad 27 has compressedits corresponding stud bump stem practically entirely its height to theupper limit of the base portion, which effectively serves as a stop forfurther compression.

EXAMPLE

[0031] In this example, the stud bump base and stem are formed of thesame material, such as gold or a gold alloy, and the relative dimensionsare selected so that the stem portion is more compliant than the baseportion. In this example, design rules suitable according to theinvention are as follows (with reference to FIGS. 3 and 4): the stem end39 diameter 33 is greater than about twice the wire diameter; and thedifference 34 (roughly the stem height) between the stud bump overallheight 35 and the base height 32 is greater than the maximum coplanarityvariation 29 of all pads 25 on the substrate 24; and the stem height 34is at least about 1.5 times the base height and typically in the rangeabout 1.5 and 3 times the base height. Optionally, the base 36 diameter31 is less than about 0.85 times the diameter of the opening 13 in thepassivation 14.

[0032] Other embodiments are within the following claims.

What is claimed is:
 1. A stud bump structure, comprising a base portion,affixed to a first member of an interconnect pair, and a stem portion,wherein the stem portion has a transverse planar top surface configuredto contact a metal pad on a second member of the interconnect pair, andwherein the stem portion is more compliant than the base portion.
 2. Thestud bump structure of claim 1 wherein the base portion comprises afirst material and the stem portion comprises a second material, and thesecond material is more compliant than the first material.
 3. The studbump structure of claim 2 wherein the first material is selected fromthe group consisting of nickel, copper, nickel alloys and copper alloys.4. The stud bump structure of claim 2 wherein the second material isselected from the group consisting of a solder, gold, and gold alloys.5. The stud bump structure of claim 3 wherein the second material isselected from the group consisting of a solder, gold, and gold alloys.6. The stud bump structure of claim 2 wherein the first materialcomprises copper and the second material comprises gold or a gold alloy.7. The stud bump structure of claim 2 wherein at least the top surfaceof the stem has a smaller diameter than the base
 8. The stud bumpstructure of claim 2 wherein the stud bump stem height is greater thanthe maximum range of noncoplanarity of all the pads on the secondmember.
 9. The stud bump structure of claim 1 wherein the bump baseportion comprises a material having a similar compliance characteristicas the material of the stem, and wherein at least the top surface if thestem has a smaller diameter than the base.
 10. The stud bump structureof claim 9 wherein the bump base portion comprises a material having thesame compliance characteristic as the material of the stem.
 11. The studbump structure of claim 1 wherein the bump base portion comprises thesame material as the stem portion.
 12. The stud bump structure of claim1 wherein at least one of the bump base portion and the bump stemportion comprises gold or a gold alloy.
 13. The stud bump structure ofclaim 9 wherein the stud bump stem height is greater than the maximumrange of noncoplanarity of all the pads on the second member.
 14. Thestud bump structure of claim 1 wherein the base portion is affixed to aconductive site exposed in an opening in a passivation material, andwherein a diameter of the base portion is less than 0.85 times adiameter of the opening in the passivation material.
 15. A method forforming a stud bump on a contact surface, comprising forming a bump baseportion on the surface, drawing out a generally conical tail from a topof the base, and truncating the tail to form a stem portion having aplanar transverse top surface and having a length from the top of thebase portion to the top surface.
 16. The method of claim 15 wherein thetail portion, at least, is formed using a wire bonding tool, and theconical tail is dimensioned so that after truncating, the resulting topsurface of the stem portion has a diameter at least about the same as aspecified wire diameter and no greater than the diameter of the base.17. The method of claim 16 wherein the conical tail is dimensioned sothat after truncating, the resulting top surface of the stem portion hasa diameter at least about twice a specified wire diameter.
 18. Themethod of claim 15 wherein the truncating includes chemical mechanicalpolishing.
 19. A method for forming an interconnect between a firstmember and a second member of an electronic package, comprisingproviding a plurality of stud bumps on contact sites at specifiedlocations on the first member, the stud bumps each having a base portionand having a stem portion that is more compliant than the base portion,the stem portion having a transverse planar top surface; providing asecond member having a plurality of bonding pads at specified locationson a surface of the second member, the respective specified locations onthe first and second members corresponding; apposing the first andsecond members such that the corresponding bumps and pads are aligned;and bringing the first and second members together under conditions thatpromote bonding of the pumps onto the respective metal pads.
 20. Themethod of claim 19 wherein one of the first and the second memberscomprises an integrated circuit chip.
 21. The method of claim 19 whereinone of the first and the second members comprises a printed circuitsubstrate.
 22. A method for making a flip chip semiconductor devicepackage, comprising providing a plurality of stud bumps on contact sitesat specified locations on an integrated circuit chip, the stud bumpseach having a base portion and having a stem portion that is morecompliant than the base portion, the stem portion having a transverseplanar top surface; providing a substrate having a plurality of bondingpads at specified locations on a surface of the substrate, therespective specified locations on the integrated circuit chip and thesubstrate corresponding; apposing the integrated circuit chip and thesubstrate such that the corresponding bumps and pads are aligned; andbringing the integrated circuit chip and the substrate together underconditions that promote bonding of the pumps onto the respective metalpads.